Electric utility power distribution systems are frequently constructed underground for a variety of reasons ranging from objections to the above-ground aesthetics, the premium of above-ground space in dense urban locations, and safety concerns. Accordingly, power distribution systems heretofore constructed of poles, wires, and pole-mounted switches and transformers are being superseded and even replaced by underground systems in underground “vaults”.
In an electric utility power distribution system, switchgear is the combination of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults down the line. Switchgear is also used to distribute power to different areas within the system. Thus, this type of equipment is important to the distribution of reliable electricity within a power system.
The size and weight of three-phase switchgear govern their installation to on-surface or underground locations. While overhead space is relatively open and unrestricted, surface space is somewhat restricted and space in underground installations is more so and at a higher premium. Thus, switchgear have dimensional restrictions imposed on them, especially for underground installations. The size of a regular switchgear using only air as an insulating medium is quite large. In order to reduce size, oil or SF6 gas was, and is, currently used in many switchgear. However, current environmental concerns discourage the use of these insulating medium. Oil and SF6 gas can be flammable and/or explosive, and present environmental problems when leakage occurs and when emissions are created.
Three-phase, two-way, vacuum interrupter switchgear have been manufactured for use in power distribution systems. The common design of these switchgear is to entirely encapsulate the vacuum bottles in a polymeric material. This design does not allow an operator to visually confirm that the switchgear is in an “open” state and may not safely contain an explosion if the switchgear closes into a fault. These safety hazards were addressed in published U.S. Patent Application No. US-2011-0253675-A1 (the content of which is hereby incorporated by reference) by adding a disconnect switch with viewing window and by encasing the vacuum bottle assemblies within a sturdy stainless steel case. The addition of a viewing window and disconnect switch to the encapsulated design does not, however, address the potential explosion hazard if the switchgear were to close into a fault.